1. Field of the Invention
The invention relates to a motor control device, an electric power steering system, and a vehicle steering system.
2. Description of the Related Art
In a system that requires high reliability and safety, such as an electric power steering (EPS) system and a vehicle steering system equipped with a transmission ratio variable mechanism, a relay is provided in a power line that connects a driving circuit that constitutes a motor control device of such a system to a power supply. For example, see Japanese Patent Application Publication No. 2009-220766 (JP-A-2009-220766). When there occurs a situation where overcurrent flows through the power line, that is, for example, when a short-circuit failure in which high-potential side and low-potential side switching elements are turned on at the same time in any one of switching arms that constitute the driving circuit (what is called the arm short circuit) has occurred, the relay is turned off to interrupt the power line so as to cut off the energization, so that it is possible to quickly achieve fail-safe.
In addition, the motor control device includes pre-drivers that respectively output driving signals (gate driving signals) to the driving circuit based on motor control signals output from a control circuit (microcomputer). In the driving circuit, the switching elements are turned on and off based on the driving signals to output driving electric powers indicated by the motor control signals. Thus, in order to ensure further high reliability and safety, it is desirable not only to protect the driving circuit by interrupting the power line as described above but also to protect the pre-drivers and power feed lines of the pre-drivers by taking similar measures against overcurrent.
As shown in FIG. 7, normally, a pre-driver 70 includes driver circuits 73 (73a to 73f) corresponding to switching elements (FETs 72a to 72f) of a driving circuit 71. Since a motor control device shown in this example is a brushless motor control device that outputs three-phase (U, V and W) driving electric power, both the number of the switching elements and the number of driver circuits are “6”. The FETs 72a to 72f that constitute the driving circuit 71 are turned on and off based on gate driving signals output from the corresponding driver circuits 73a to 73f. 
In most cases, each driver circuit 73 includes a push-pull circuit 74 at its output stage. In this example, the push-pull circuit 74 is used, in which a p-channel FET 75 is arranged at a high-potential side and an n-channel FET 76 is arranged at a low-potential side. The push-pull circuit 74 inverts the output of an operational amplifier circuit (inverting amplifier circuit) 77 that is a driver unit of the driver circuit 73, and then outputs the inverted output of the operational amplifier circuit 77 to a corresponding one of the FETs 72a to 72f of the driving circuit 71.
That is, each driver circuit 73 of the pre-driver 70 amplifies the motor control signal based on the applied voltage to thereby output the gate driving signal to a corresponding one of the FETs 72a to 72f of the driving circuit 71. Thus, for example, if a short-circuit failure occurs in any one of the push-pull circuits 74, overcurrent may flow through the pre-driver 70 and the power feed line thereof as in the case where a short-circuit failure occurs in the driving circuit 71 as described above.
However, unlike the driving circuit 71 and the power line thereof, in the case of the pre-driver 70 and the power feed line thereof, large current does not flow basically while the pre-driver 70 and the power feed line operate normally. Thus, it is extremely less likely that a short-circuit failure occurs inside the pre-driver 70. For this reason, in the existing art, it is less necessary to take measures against overcurrent in the pre-driver and the power feed line thereof.
In the meantime, there is a motor control device that is configured to apply a voltage, stepped up by a step-up circuit, to a pre-driver. For example, see JP-A-2009-220766. By employing a configuration, in which a step-up voltage is applied to the pre-driver, for example, the response of an EPS is improved in a case where an energization direction is switched, for example, when the right or left steering direction is reversed.
However, by increasing the applied voltage as described above, the possibility increases that a short-circuit failure occurs inside the pre-driver. Furthermore, when a smoothing capacitor is provided at the output stage of the step-up circuit, a possibility additionally occurs that a ground fault occurs in the capacitor. Thus, in the above configuration, in which a step-up voltage is applied to the pre-driver, the possibility that overcurrent occurs in the pre-driver is non-negligible, and there is a strong need for effective measures against overcurrent.